contaminated MLR

Dear listers,
doing primary allogeneic MLRs (murine)I faced the problem of bacterial
contamination in all wells which included T cells plus BM dendritic cells
at day3; there was no detectable (by microscope) contamination of wells
with T cells or dendritic cells alone. There was also no contamination in
wells with the same T cells plus total splenocytes.
What could be the explanation?
Sebastian
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